NCDC added Alaska climate divisions to its nClimDiv dataset on Friday, March 6, 2015, coincident with the release of the February 2015 monthly monitoring report. For more information on this data, please visit the Alaska Climate Divisions FAQ.

National
Overview

Based on the Palmer Drought Index, severe to extreme drought affected about 18 percent of
the contiguous United States as of the end of August 2006, a
decrease of about 9 percent
compared to last month. By contrast, about 5 percent of the
contiguous U.S. fell in the severely to extremely wet
categories.

On a broad scale, the previous two decades (1980s and 1990s)
were characterized by unusual wetness with short periods of
extensive droughts, whereas the 1930s and 1950s were characterized
by prolonged periods of extensive droughts with little wetness
(moderate
to extreme drought,
severe to extreme drought).

Historical temperature, precipitation, and Palmer drought data
from 1895 to present for climate divisions, states, and regions in
the contiguous U.S. are available at the Climate Division:
Temperature-Precipitation-Drought Data page in files having
names that start with "drd964x" and ending with "txt" (without the
quotes).

Drought impacted many sectors of the economy. Crops were highly
stressed or dying, livestock was dying or prematurely sold because
of a lack of feed and water, and water restrictions were common in
many areas. Disaster conditions have been declared by the governors
of several states. The USDA plans to distribute about $100,000,000
to farmers and ranchers. Companies whose sales indirectly depend on
precipitation, such as lawn mower manufacturers, have partially
shut down because of reduced demand. Low water in the Mississippi
River affected transportation of farm products, petroleum, steel
and ore. Impacts in drought-stricken areas have been collected and
summarized by county at the National Drought Mitigation Center's
Drought Impact Reporter.

Highly stressed corn field east of Kansas City (photo courtesy of
University of Missouri Extension)

Empty Platte River (photo courtesy of the High Plains Regional
Climate Center)

The August precipitation pattern at the primary stations in
Alaska was above normal across most of the state; the northwest
coastal stations were below normal. In Hawaii
dryness continued throughout most the State. In Puerto
Rico the month was predominantly dry in the central and eastern
interior areas and along the southern coast (based on National
Weather Service radar estimates of precipitation).

The graph below shows
the summer (June-August) PDSI, 1896-2006, for Texas Division 3
(annual values in light blue, 5-year weighted average in dark
blue). The value for 2006 is the lowest since 1956 (-5.91), which
was the most severe summer drought on record. The most persistent
drought was in the 1950s, when summer PDSI was negative (in fact,
below -2) for six years in a row (1951-1956; yellow arrow).

The graph to the left
also shows a 329-year tree-ring reconstruction (1650-1978) of
summer PDSI for a gridpoint (32.5 N, 97.5 W) within Texas Division
3 (annual values in light red, 5-year weighted average in dark
red). The reconstruction is one of over 200 developed by Cook et
al. (1999, 2004) across a gridded network covering much of North
America, and is based on tree-ring data from 32 sites in Texas and
surrounding states. The correlation between reconstructed gridpoint
PDSI and Division 3 instrumental PDSI over their common period
(1896-1978) is 0.810, indicating a very high degree of shared
variance.

The tree-ring
reconstruction can put the droughts of the last century in north
central Texas into a much longer perspective. The frequency of
severe one-year droughts appears to have remained fairly constant
over the past ~350 years. Both the instrumental and reconstructed
PDSI records indicate that "severe" or "extreme" summer drought
(PDSI below -3) occurred in about 1 in 9 years. "Extreme" summer
drought (PDSI below -4) such as 2006 is seen in about 1 in 20 years
in both the instrumental and reconstructed records.

The severe and
sustained 1950s drought appears to be unusual even in the context
of the multicentury tree-ring record. From 1951 to 1956, the
average instrumental summer PDSI was -3.60 (the average
reconstructed summer PDSI was -3.16). The closest analog in the
paleorecord is the six-year drought 1859-1864 (orange arrow), for
which the average reconstructed summer PDSI was -2.87. There is one
seven-year drought in the paleorecord, from 1772-1778 (red arrow),
with an average reconstructed summer PDSI of -2.07.

The most notable
aspect of the current drought was the profound dryness in fall and
early winter; precipitation from September-December
2005 in Division 3 was only 24% of normal, the lowest level in
the 111-year instrumental record. This extreme dryness led to the
spectacular grass fires in Oklahoma and Texas in December and
January. But this spectacle aside, the instrumental and tree-ring
records of PDSI show the 2006 drought to be an infrequent, but not
extraordinary, event.

Resources:

Divisional climate data, including precipitation for Texas
Division 3 as shown above, can be obtained from NCDC.